The present invention relates to a system for controlling the air-fuel ratio for an internal combustion engine having an emission control system with a three-way catalyst and more particularly to a system for controlling the air-fuel ratio during the idling and heavy load operations at a high altitude for improving the fuel consumption and emission control.
Such a system as in U.S. Pat. No. 4,132,199 comprises a feedback control system, in which an oxygen sensor is provided to sense the oxygen content of the exhaust gases to generate an electrical signal as an indication of the air-fuel ratio of the air-fuel mixture supplied to the engine. The control system operates to actuate an air-fuel mixture supply means with a duty ratio correcting means to control the air-fuel ratio of the mixture of the stoichiometric air-fuel ratio in response to the signal from the oxygen sensor. In such a system, the feedback control with the oxygen sensor is not operated during the idling and heavy load operations of the engine and the system acts in a manner so as to operate the air-fuel mixture supply means with a predetermined duty ratio, in order to stabilize the idling operation respectively and to perform a high power operation. However, the duty ratio of the supply means is adjusted so as to supply a mixture having an air-fuel ratio sufficient to improve the operation at a low altitude. Therefore, when the air-fuel mixture supply means is fixed to the predetermined duty ratio at a high altitude during idling and heavy load operations, the air-fuel ratio of the mixture supplied by the supply means decreases because of a decrease in the oxygen concentration in the air. Thus, a rich air-fuel mixture is supplied to the engine, which will result in disadvantageous fuel consumption and emission control effect.